This invention generally relates to a system and method for producing halftoned color separations from input data describing an image and, more particularly, to a system and method for producing halftoned color separations that are ideally suited for a specific output imaging device.
Traditionally, in the prepress field, input data describing an image, such as a PostScript input file, is transformed into at least one halftoned color separation which is ultimately either saved to a file or immediately sent to an output device for imaging. The transformation of the input data to at least one halftoned color separation is performed by a Raster Image Processor (RIP).
In particular, the RIP converts input data to bitmap data, which is required by output devices to render the image. In this regard, the RIP creates a bitmap image, i.e. a series of 1s and 0s, typically for each of the colorants used to compose the image, and hence, in this instance, the resulting bitmap is also called a xe2x80x9chalftoned color separationxe2x80x9d which is a 1 bit per pixel bitmap. When the input data is completely rasterized, the RIP transfers the bitmap data via a device driver to an output device which uses the information in the bitmap data to render the image. Alternatively, the RIP can store the bitmap data to a file for later importation into an output device.
Typically, if the input data identifies xe2x80x9cNxe2x80x9d colorants for composing the image, most RIPs receiving such input data produce xe2x80x9cNxe2x80x9d halftoned color separations, one for each colorant. However, a selected output device may only be capable of imaging xe2x80x9cMxe2x80x9d halftoned color separations such that there is not a direct mapping from the N halftoned color separations to the M halftoned color separations. In order for a direct mapping to exist, all of the N colorants must be included in the set of colorants corresponding to the M halftoned color separations.
As an example, if N is greater than M and the N halftoned color separations include the specific set of M halftoned color separations, the output device images only the M halftoned color separations. Thus, the resulting image lacks any colors provided by the other of the N halftoned color separations which were not included in the set of M halftoned color separations and hence were not imaged by the output device. In particular, assume that the RIP produces five halftoned color separations representing process colors cyan, yellow, magenta and black (CYMK) and a spot color xe2x80x9ccherry apple red.xe2x80x9d Also, assume that the output device is capable of imaging four halftoned color separations representing the process colors CYMK. In this case, the selected output device will only image the four halftoned color separations representing the process colors, i.e. the colors in common between the sets of halftoned color separations. Thus, any objects in the image described in the spot color will not be provided in the output from the imaging device.
As another example, if N is equivalent to M and the specific halftoned color separations produced by the RIP and required by the output device are identical, i.e. there is a direct mapping from the N halftoned color separations to the M halftoned color separations, the result from the output device most accurately describes the original image since all of the halftoned color separations provided by the RIP are imaged by the output device. However, in this example, if the specific halftoned color separations are not identical, the output device will only image those colors represented by halftoned color separations that the output device has in common with the ones provided by the RIP.
Some RIPs try to resolve the negative results described above by internally converting the halftoned color separations that it produces into the appropriate number and set of halftoned color separations for a select output device. However, presently, when these converted color separations are applied to the output device, the output therefrom does not accurately reflect the original content used to compose the image. Some colorants may be missing or misrepresented with respect to overprints or knockouts.
As a result, there is a need for a system and method that produces the appropriate number and set of halftoned color separations for a select output device such that the resulting image more accurately reflects the original.
In accordance with the present invention, a raster image processing system automatically produces the appropriate number and set of halftoned color separations for a select output device. The raster image processing system includes a processing unit, an interpreter and a color combiner. The interpreter and color combiner are application processes that are controlled by software running on the processing unit. The processing unit receives input data describing an image having xe2x80x9cNxe2x80x9d colorants and controls processes implemented by the interpreter and the color combiner. The processing unit identifies the number and set of colorants used to describe input image data and the number and set of halftoned color separations that an output imaging device is capable of rendering. The interpreter converts the input data describing an image having xe2x80x9cNxe2x80x9d colorants into xe2x80x9cNxe2x80x9d halftoned color separations. Then, the color combiner converts the N halftoned color separations into xe2x80x9cMxe2x80x9d halftoned color separations that are ideally suited for a particular output device as chosen by a user of the system. The color combiner performs this conversion by utilizing digital filtering and combining processes.
Before filtering and combining the halftoned color separations produced by the interpreter, the raster image processing system performs a database search to identify a set of filters to apply to each of the N halftoned color separations. In particular, the raster image processing system identifies tint percentages for each of the N halftoned color separations. The tint percentages identify the percentage contribution of each color that the output device is capable of imaging in each of the N halftoned color separations. Then, the raster image processing system dynamically computes the filters based on the tint percentages. The filters are bitmaps that, once created, are applied to the N halftoned color separations by performing a bitwise xe2x80x9cANDxe2x80x9d combination function therebetween to produced filtered halftoned color separations. Then, select filtered halftoned color separations are combined using a bitwise xe2x80x9cORxe2x80x9d combination function such that the appropriate M halftoned color separations are produced. These M halftoned color separations are ideally suited for the selected output device to most accurately render the original image.
As will be readily appreciated from the foregoing description, the invention provides for a raster image processing system that produces M halftoned color separations that are ideally suited for the selected output device to most accurately render the original image. Such a raster image processing system is capable of being used with numerous different output devices, each of which is capable of imaging a different number and/or types of colorants, and still produce the same image because the M halftoned color separations produced by the system are tailored to the specific output device selected by a user.